1. Field of the Invention
The present invention relates to an information transmission system for transmitting a digital information signal from a sender to a receiver, and more particularly, to an information transmission system and an information signal transmission apparatus both having functions to diagnose an information signal for an error and correct the error if any.
2. Description of Background Information
There is a well-known information transmission system which multiplexes digital information signals of different types along the time axis, and transmits the multiplexed signal to a receiver from a sender. In particular, it is a recent trend to permit multiple users to share a single line, like resales of communication lines. What is important in such multiplexed transmission is to prevent information signals of different types from being mixed with one another. As a solution to such mixing, information signals of the same type may be consecutively transmitted from the sender to the receiver. With an increase in the amount of information of the same type, however, the information transmission would occupy the line for a long time, disabling the use of the line for transmission of an information signal with a small amount of other information or an emergency information signal while the line is busy. This impairs the efficient use of the communication line.
Recently an information transmission system, such as a CATV system, which transmits an information signal from a base station to a relay station and retransmits the information signal from that relay station to terminals has become popular. Also known is a method of transmitting an information signal affixed with a redundancy code for error correction. It is however difficult for the error correct system to correct communication errors completely. If many errors occur during transmission, it is necessary to take some measures against the errors. There are many one way addressable (OWA) systems particularly among information transmission systems utilizing a communications satellite for signal transmission from the base station to a relay station due to various restrictions involving the equipment cost, the number of channels of satellite communication lines, etc. This is a disadvantage in that the base station cannot acquire the necessary diagnosis data when needed.
Further, there is proposed an information transmission apparatus which comprises a base station for separating a digital information signal into packets each carrying a predetermined amount of information, and transmitting the packets of information with an error correct code bit affixed to each divided information signal, and multiple relay stations for receiving the information signals. Each packet has the error correct code bit for correcting errors affixed to the end and an identification (ID) code including relay station pointing codes for the individual relay stations affixed to the head portion or header.
Each receiving station decodes the relay station pointing code in the ID code in each packet to determine whether or not that packet is addressed to itself. Since the error correct code bit is affixed to the end of each packet, however, any error correction for that packet is made after the whole packet of information is received. The receiving station should therefore pick up all the packets from the base station irrespective of the relay station pointing code in the ID code, thus impairing the efficiency of reception in each receiving station.